Field of the Invention
The invention concerns a method for determining an echo time that is as minimum as possible for a radio-frequency coil used in a scanner of a magnetic resonance apparatus, and a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
The advantages of magnetic resonance sequences with “ultrashort” echo times (TE) are widely discussed. Ultrashort echo times are understood in the art as meaning echo times that are shorter than 500 μs. Magnetic resonance sequences with ultrashort echo times open new potential applications for magnetic resonance imaging, since they also enable magnetic resonance signals of substances that have very short T2 or T2* relaxation times to be measured and displayed. For example, bones, tendons, ligaments, teeth, lung tissue, etc., can be visualized. These cannot be captured with conventional magnetic resonance sequences that usually make use of an echo time of significantly more than one millisecond, so that the magnetic resonance signal of these materials/substances has already decayed at this point in time. Besides the display of tendons, ligaments and bones in orthopedics, magnetic resonance sequences with ultrashort echo times are also employed for creating a μ map for the MR/PET attenuation correction in combined MR/PET imaging devices.
Typical examples of magnetic resonance sequences with ultrashort echo times are the UTE sequence (“ultrashort TE”), the PETRA sequence (“Pointwise Encoding Time reduction with Radial Acquisition”) and the zTE sequence (“zero TE”). In all these magnetic resonance sequences, the minimum possible echo time is dictated by the time that the hardware requires to switch between transmit and receive operation, called the dead time Tdead. This is because the radio-frequency coils must be detuned when the excitation pulse is emitted and this detuning must first be deactivated again. In known standard, clinical radio-frequency coils the dead time is about 40 μs. Since the spins effectively relax from the midpoint of the excitation pulse, the minimum possible echo time is obtained by adding half the duration of the excitation pulse to the dead time. If the duration of the excitation pulse is 60 μs, for example, and the dead time is 40 μs, the minimum possible echo time is TEmin=70 μs.
The dead time can vary from one magnetic resonance device to another, as well as from one radio-frequency coil to another. It can also occur that radio-frequency coils can also work with dead times significantly less than the nominal dead time that is actually specified. Dependencies on the flip angle that is employed are also possible. However, if the user chooses an echo time that is too short, the first points of the readout operation are not measured correctly, resulting in image artifacts. To prevent this, it is known to permit only one single minimum possible echo time, which can be set, in hardcoded form, for all radio-frequency coils.
However, this means that in most cases the actual possible minimum echo time cannot be set, resulting in a poorer signal-to-noise ratio for substances with an extremely short T2 relaxation time, as well as a longer measurement time for example in the PETRA sequence, since in that case a larger central portion of k-space has to be captured in Cartesian form. By providing minimum possible echo times that should work with a sufficient buffer for all radio-frequency coils, it may nevertheless occur that when using older radio-frequency coils the minimum possible echo time is not long enough to prevent image artifacts.